The present invention is directed to methods and apparatus for joining sections of double-wall steel pipe. Double-wall steel pipes are typically used in specialty pipeline systems to provide an additional measure of protection for containment of the material being transported. For instance, double-wall steel pipelines are typically installed in subsea operations and are often used for the transportation of corrosive or hazardous material. The annulus between the two "walls" may be filled with an insulation material to provide thermal protection for temperature-sensitive materials, such as crude oil or heavy fuel oil, where external pipe insulation methods are not suitable. In some applications, the combination of both leak containment and thermal protection are desirable.
A double-wall steel pipe typically comprises a steel outer casing and a steel inner carrier pipe. The outer casing provides bending, and tensile strength to the pipe, in addition to providing containment of the material being transported. The inner carrier pipe provides bending and tensile strength, as well as radial strength.
Historically, sections of double-wall steel pipes have been joined by welding both the inner carrier pipe and the outer casing. The inner carrier pipe must extend beyond the outer casing on both ends of adjacent double-wall sections in order to provide access for full circumference welding and weld inspection. Welding is done either by a machine affixed to the inner carrier pipe or by hand. After the sections of inner carrier pipe are welded, a two-piece steel joint is used to join the steel outer casing. This two-piece casing joint is comprised of two half-rings of sufficient length to join adjacent sections of outer casing. The half-rings are fabricated from a section of steel pipe with the same outside diameter, wall thickness and steel grade as the steel outer casing. The steel pipe section is cut to fit the open area remaining between adjacent outer casing sections. The two half-rings are formed by cutting the steel pipe section axially 180.degree. apart. One half-ring forms the bottom portion of the casing joint, the second half-ring the top portion. Two axial welds join the half-rings to form the outer casing joint. Two full circumference welds join adjacent sections of steel outer casing to the steel casing joint. This method of joining two sections of double-wall steel pipe is both time-consuming and labor-intensive.
The present invention eliminates the step of welding in order to join the inner steel pipe. In addition, the present invention succeeds in joining the steel outer casing of adjacent double-wall pipe sections with one weld instead of four. Reducing the number of welds required saves time and increases production.
The disadvantages of the prior art are overcome with the present invention, and novel methods and apparatus are hereinafter described for efficiently and reliably joining together sections of double-wall steel pipelines.